Foreign Secretary Boris Johnson has said Britain will not "walk away" from the Iran nuclear deal following the dramatic withdrawal of the US from the agreement.

European diplomats were scrambling to salvage the three-year-old accord amid fears that Donald Trump's threat to impose the "highest level" of sanctions on Tehran could trigger a new confrontation in the region.

Chancellor Angela Merkel said Germany remained committed to the agreement while the French foreign minister, Jean-Yves Le Drian, insisted it was not "dead".

"For as long as Iran abides by the agreement ... then Britain will remain a party to the JCPOA. Britain has no intention of walking away," he said.

"Instead we will co-operate with the other parties to ensure that while Iran continues to restrict its nuclear programme, then its people will benefit from sanctions relief in accordance with the central bargain of the deal."

In a statement, the International Atomic Energy Authority - the global watchdog responsible for monitoring the agreement - said "as of today" Tehran was continuing to honour its commitments.

Mr Johnson's comments came amid fears that the reimposition of US sanctions could hit European firms - which have led the way investing in Iran under the JCPOA - particularly hard.

Earlier Foreign Office minister Alistair Burt said there was a need to "de-escalate tensions" after President Hassan Rouhani warned Iran could restart enriching uranium - a key element of a nuclear weapons programme - "without any limitations" within a matter of weeks.

With tensions already running high, Mr Burt sought to play down suggestions that the US was ready to launch an attack on Iran, possibly using "proxies" such as the Israelis or the Saudis, who also remain highly suspicious of Tehran's nuclear ambitions.

He acknowledged however that the US president's decision - despite appeals from Theresa May, French President Emmanuel Macron and German Chancellor Angela Merkel not to abandon the accord - had shown the limits of European influence in Washington.

"On this issue he has not listened," Mr Burt told the BBC Radio 4 Today programme.

"We won't be seeking to make him go back on something. He won't do that.

"But there are other ways forward and it is our job to make sure those other ways work and work in a non-confrontational fashion, no matter how difficult that is in a tricky region."

Mr Le Drian said he and his British and German counterparts would meet Iranian representatives on Monday to discuss the next steps.

.@POTUS Trump to the long-suffering people of #Iran: The people of America stand with you. The future of Iran belongs to its people. They deserve a nation that does justice to their dreams. #JCPOApic.twitter.com/MMeIpZEG8p

Speaking in the White House on Tuesday, Mr Trump said the agreement - seen as the key diplomatic legacy of his predecessor Barack Obama - was "defective at its core".

"If we do nothing, we know exactly what will happen. In just a short period of time, the world's leading state sponsor of terror will be on the cusp of acquiring the world's most dangerous weapon," he said.

In a joint statement, Mrs May, Mrs Merkel and Mr Macron expressed their "regret" at the US decision while emphasising their continuing commitment to the JCPOA.

The agreement with Iran was signed by the US, China, Russia, Germany, France and Britain in 2015 following years of tortuous negotiations.

Under its terms, Iran agreed to scale back key elements of its nuclear energy programme associated with the development of a nuclear weapon in return for the easing of economic sanctions.